Disjoint stationary sets that reflect Hello, I have the following question (for definitions see at the end):
Let $\kappa$ be an uncountable regular cardinal. 
Can we prove in ZFC that there exist two disjoint stationary sets $A$, $B$ such that for every limit ordinal $\alpha<\kappa$ of uncountable cofinality, both $A$ and $B$ reflect at $\alpha$?
Definitions: (1) $A$ is a stationary set on $\kappa$, if $A\subset\kappa$ and $A$  intersects every closed and unbounded set in $\kappa$. 
(2) A set is closed if it contains its limit points.
(3) A stationary set $A\subset\kappa$ reflects at $\alpha$ if $A\cap\alpha$ is stationary on  $\alpha$.
 A: A general affirmative answer is possible if one assumes the global square principle, which holds in $L$ and in many other canonical models. Indeed, the failure of $\square$ is a strong hypothesis. 
Definition. The global square principle $\square$ is the
assertion that there is an assignment $\nu\mapsto C_\nu$ for all
singular ordinals $\nu$, such that


*

*$C_\nu$ is a closed subset of $\nu$, containing only singular
ordinals;

*If $\nu$ has uncountable cofinality, then $C_\nu$ is unbounded
in $\nu$;

*the order type of $C_\nu$ is less than $\nu$;

*and if $\mu\in C_\nu$, then $C_\mu=C_\nu\cap\mu$.


(For reference, see definition 19 of Square in Core Models, by Schimmerling and Zeman, or numerous other accounts.) 
Theorem. If the global square $\square$ principle holds,
then the answer to the question is yes, every $\kappa$ has such a partition.
Indeed, under $\square$ there is a coherent global partition of the class of singular
ordinals into $A\sqcup B$, such that for every $\kappa$ of
uncountable cofinality, both $A\cap\kappa$ and $B\cap\kappa$ are
stationary in $\kappa$.
Proof. Fix the $\square$ sequence $C_\nu$. First, define $A$ and $B$ up to
$\omega_1$ to be any partition of the singular countable ordinals
into stationary sets. Suppose now that $A$ and $B$ are defined up
to $\nu$, a singular limit ordinal. Consider $C_\nu$, which has
some order type $\eta<\nu$. If $\eta\in A$, then put $\nu\in A$,
otherwise, put $\nu\in B$. Continue by transfinite
recursion. Note that $A$ and $B$ partition the singular ordinals.
Suppose that $\kappa$ has uncountable cofinality. If
$\kappa=\omega_1$, then $A\cap\kappa$ and $B\cap\kappa$ are the
stationary sets that we used to start the construction. More
generally, if $\kappa\gt\omega_1$ but has cofinality $\omega_1$,
then $\kappa$ is singular and so $C_\kappa$ is a club of some type
$\beta<\kappa$. Further, $A$ and $B$ when restricted to $C_\kappa$
are copies of $A\cap\beta$ and $B\cap\beta$, which by induction
are each stationary. So $A\cap\kappa$ and $B\cap\kappa$ are
stationary. Finally, we have the case that $\kappa$ has cofinality
larger than $\omega_1$. Fix any club $C\subset\kappa$. Thus, there
is some singular $\eta\in C$ with uncountable cofinality. So
$C_\eta\cap C$ is club in $\eta$ and thus meets both $A$ and $B$,
and so $C$ meets both $A$ and $B$, as desired. QED
Since $\square$ holds in $L$, this means that ZFC+V=L proves the
affirmative answer.
(Click on the edit history to see my original answer, which handles just the case for $\kappa=\omega_2$, assuming $\square_{\omega_1}$. The idea here follows something like the idea of Eran's construction, but seems to require $\square$ in order to avoid the incoherence issue mentioned by Andreas in the comments.)
A: In the presence of large cardinals, one can (or rather Shelah can...) force the answer to be "NO" in a very strong sense.  The place to look is Section 7 of Chapter X of Proper and Improper Forcing.
In particular, Theorem 7.4 shows that assuming the consistency of 2 supercompact cardinals, one can force that for any regular $\kappa>\omega_1$, any stationary subset of $S^\kappa_{\aleph_0}$ contains a closed copy of $\omega_1$.
This implies the answer to your question is no by the following argument: 
Step 1:  If $\kappa>\aleph_1$ is regular and $A$ reflects at all uncountable limit ordinals below $\kappa$, then so does $A\cap S^\kappa_{\aleph_0}$ (where $S^\kappa_\tau$ is the set of ordinals less than $\kappa$ of cofinality $\tau$).
Proof: Let $A_0= A\cap S^\kappa_0$, and let $A_1= A\setminus A_0$.  $A_1$ cannot reflect at ordinals of cofinality $\omega_1$, and so it must be the case that  $A_0$ reflects at all ordinals of cofinality $\omega_1$. But then $A_0$ also reflects at any place where $S^\kappa_{\aleph_1}$ reflects as well, and so $A_0$ reflects at all ordinals of uncountable cofinality below $\kappa$.
Step 2:
Assume we are in a model like that obtained by Shelah.  If $\kappa$ is a regular cardinal greater than $\aleph_1$ and $A$ is a stationary subset of $S^\kappa_{\aleph_0}$.  We know $A$ contains a closed copy $C$ of $\omega_1$, and if we set $\delta=\sup(C)$ then $\delta$ is an ordinal of cofinality $\omega_1$ where $A$ reflects but $\kappa\setminus A$ does not.  In particular, no stationary subset disjoint to $A$ can reflect at $\delta$, hence there is no way to get your "$B"$.
Edit:
A "no" answer to your question at $\omega_2$ is equiconsistent with the existence of a Mahlo cardinal.
As Joel mentioned in (an earlier version of) his answer, one can build $A$ and $B$ in $\omega_2$ from a $\square_{\omega_1}$-sequence.  The failure of $\square_{\omega_1}$ implies that $\aleph_2$ is Mahlo in $L$  (Credited to Jensen on page 453 of Jech's "Set Theory"; I don't know a better reference.)
On the other hand, Theorem 7.1 in Chapter XI (page 576) of Proper and Improper forcing tells us that from a Mahlo cardinal, we can force ZFC+GCH + "every stationary subset of $S^{\omega_2}_{\omega}$ contains a closed copy of $\omega_1$, which we argued above gives a "No" answer.
Note that what Shelah is really showing is the consistency of the following statement:
"If $S$ is a stationary subset of $S^{\omega_2}_{\omega}$ that reflects at every member of $S^{\omega_2}_{\omega_1}$, then $S^{\omega_2}_{\omega}\setminus S$ is non-stationary,"
while the original question is equivalent to asking of $S^\kappa_\omega$ can be partitioned into two disjoint stationary sets, each of which reflects at every ordinal in $S^\kappa_{\omega_1}$.
A: Let's take an example - $\kappa = \omega_2$.
The set $D$ of all ordinals less that $\omega_2$ with cofinality $\omega_1$ is stationary in $\omega_2$.
Split it to two stationary sets A and B (using Solovay's theorem?).
Now take for each ordinal in these sets an $\omega_1$ cofinal series, and split it (based on even and odd indices), between the two sets.
I believe A and B now provide the requirement.
